This application is a national stage filing under 35 U.S.C. xc2xa7371 and priority is hereby claimed on International Application No. PCT/GB98/02039, Filed Jul. 10, 1998, which International Application was published in English as No. WO 99/02363.
The present invention relates in general to brake control in apparatus comprising an electric motor arranged to drive a wheel. In particular, although not exclusively, the present invention relates to electric vehicles, and is especially relevant to brushless motor-driven wheelchairs controlled by the user solely by means of a joystick.
The joystick is usually the only means by which the user can control an electric wheelchair. The wheelchair is often fitted with a static brake, that is a brake which ideally is to be applied only when the wheelchair is stationary, to hold the chair and prevent undesired further movement after coming to rest on level or, more importantly, sloping ground.
In the case of wheelchairs in which the wheels are driven directly by brushless dc motors, the static brake is particularly important because of the absence of gearing which would increase rolling resistance and assist in holding the chair when the motor windings were not excited. It is important that the motor controller connected to the joystick can operate the static brakes satisfactorily as the user normally has no direct control over them. Ideally, the static brake should come on immediately-and automatically after the chair has been dynamically slowed by the motors and stopped. Good brake timing prevents rolling backwards or forwards after slowing down on gradients. It also reduces wear on the brakes.
A problem is that on a brushless system often there is no speed feedback of sufficient quality to tell the motor controller when to apply the static brakes (ie. tell the controller that the wheels are revolving slowly enough).
Ideally one would like to monitor the back EMF of the motor, which would of course fall to zero when the motor stopped. However, when the motor is slowing down the voltage measured across its terminals is equal to the back EMF plus IR (where I is the current flowing through the motor and R is the resistance of the motor). Thus, if one tries to calculate the back EMF from this measured voltage for use as a type of speed feedback there are three sources of error: the measurement of the voltage across the motor; the measurement of the current flowing through the motor windings; and the assumed resistance of the motor. The resistance of the motor can vary with temperature, and IR becomes large compared to the back EMF at low speeds when dynamically braking. Accordingly, speed feedback values will become increasingly inaccurate at low speeds. Unfortunately, this is at exactly the point one would like an accurate measure of the speed for the correct timing of static braking.
To add quality speed feedback sensors would be expensive, and so, in the past, other indirect techniques have been employed to guess at the wheelchair""s velocity and apply the static brake at an appropriate time.
One of these techniques has been to apply the static brake a fixed time interval after the motor control voltage (set according to signals from the joystick, and which will also be referred to as the demand voltage) has fallen to zero. A disadvantage of this technique is, however, that a fixed time interval suitable for use when the chair is stopping on level ground may be too long when the vehicle is stopping on an uphill slope, resulting in rollback, and too short when the vehicle is stopping on a downhill slope, resulting in the static brake being applied when the chair is travelling at inappropriately high speed, causing wear on the brakes and possible skidding.
It is known to set the fixed time interval according to parameters such as the weight of the user and the weight of the chair, but the problem of ensuring correct static brake application on a variety of slopes remains.
It is an object of embodiments of the present invention to provide apparatus comprising an electric motor arranged to drive a wheel with improved brake control.
It is a further object of embodiments of the present invention to provide an electric vehicle with improved brake control.
It is a further object of embodiments of the present invention to provide an improved brake control method.
According to a first aspect of the present invention there is provided brake control apparatus comprising:
an electric motor arranged to drive a wheel;
brake means operable to apply a braking force to inhibit the rotation of said wheel;
input means operable to generate an input signal indicative of a desired angular velocity of said wheel;
current monitoring means for generating a monitoring signal indicative of a current flowing through said electric motor; and
control means, for controlling the supply of power to said electric motor in response to said input signal by generating a control voltage, and for controlling said brake means such that after setting said control voltage to zero said brake means apply said braking force at a braking time determined according to said monitoring signal.
By determining the braking time according to the monitoring signal, which in turn is determined by the motor current, the control means is able to control the brake means to apply the braking force to the wheel at an appropriate time when the wheel is stopping in a variety of load configurations.
The apparatus may be comprised in an electric vehicle, providing the advantage that the control means is able to control the brake means to apply the braking force to the wheel at an appropriate time when the vehicle is stopping on a variety of slopes, thus reducing wear on the braking means and rollback or skidding.
Advantageously the braking time may be determined according to the magnitude of the monitoring signal when the control voltage is set to zero. In the case of electric vehicles, for a range of slopes, the motor regeneration current at this time has been found to be a good indication of the time that the vehicle would take to come to a halt under dynamic braking on the particular slope, after the zero demand point. This current has also been found to be largely independent of the vehicle speed at the start of the stopping process.
The control means may comprise a microprocessor, and the control voltage and braking time may be calculated according to the input signals and monitoring signal respectively.
The control unit may be operable to control the brake means to apply the braking force a delay time interval after setting the control voltage to zero, this delay time interval being determined by the monitoring signal.
The delay time interval may be determined according to the magnitude of the monitoring signal substantially at the time when the control voltage is set to zero, and this delay time interval may be increased as the magnitude of the monitoring signal at this time increases over at least a range of values.
Again, in the case of electric vehicles, the motor regeneration current at this time has been found to be a good indication of the time that the vehicle would take to come to a halt under dynamic braking on a particular slope, and by increasing the delay time interval with increasing monitoring signal the control means is able to operate the brake at a time appropriate to the slope.
In embodiments of the present invention where the control unit comprises a microprocessor, the delay time interval may be calculated according to the monitoring signal, and advantageously may be calculated according to an algorithm which includes the step of calculating a quantity which is proportional to the magnitude of the motor current when the control voltage is set to zero, i.e. at the time the control voltage substantially reaches zero in response to the input signal having reached a value indicative of the user wishing to stop the rotation of the wheel.
Of course, the delay time interval may also be dependent on the sign of the monitoring signal.
The algorithm may include the step of calculating a quantity which is linearly dependent on the magnitude of the monitoring signal at the time when said control voltage is set to zero.
Advantageously, for safety, the delay time interval may have a predetermined maximum length.
In further embodiments, the braking time may be determined according to the rate of change of the monitoring signal. Thus, after the control voltage has fallen to zero, the control unit may monitor the rate of change of the motor current, and operate the brake at a braking time determined according to the rate of this change. This enables improved brake application timing to be achieved, as it has been found that there is a distinctive change in motor current at the end of its deceleration period, i.e. just as the wheel is coming to rest.
Advantageously, the braking time may be determined according to the time at which the rate of change of the monitoring signal reaches a predetermined value.
The braking time may be substantially the time at which the rate of change of the monitoring signal reaches the predetermined value, or alternatively may be set a predetermined time interval after this time.
The predetermined value and/or the predetermined time interval may be preprogrammed.
Advantageously, the braking time may be determined according to the polarity of the monitoring signal, and may be substantially the time at which the monitoring signal changes polarity.
Advantageously, the control unit may be operable to control the brake means to apply the braking force a predetermined maximum time delay after the control voltage has fallen to zero.
The value of the control voltage at a particular time may be dependent on previous input signals, and the control voltage may be determined by smoothing and filtering the input signals.
The electric vehicle may be an electric wheelchair, and the input means may comprise a joystick.
The control means may be operable to apply a drive voltage across the windings of the electric motor, this drive voltage being determined by the control voltage. The drive voltage may be proportional to the control voltage and may be pulse width modulated.
Advantageously, the electric motor may be a brushless dc electric motor and may have an external rotor connected directly to the wheel.
The electric motor and the brake means may be integral.
According to a second aspect of the present invention there is provided an electric vehicle comprising:
a wheel;
an electric motor arranged to drive the wheel;
brake means operable to apply a braking force to inhibit the rotation of said wheel;
input means operable to generate an input signal indicative of a desired angular velocity of said wheel;
slope monitoring means for generating a monitoring signal indicative of the slope of the ground on which the vehicle is standing; and
control means, for controlling the supply of power to said electric motor in response to said input signal by generating a control voltage, and for controlling said brake means such that after setting said control voltage to zero said brake means apply said braking force at a braking time determined according to said monitoring signal.
According to a third aspect of the present invention there is provided a method of controlling brake means for applying a braking force to inhibit the rotation of a wheel driven by an electric motor, the method comprising the steps of:
setting a control voltage for controlling said electric motor according to an input signal indicative of a desired angular velocity of said wheel;
monitoring a current in said motor;
setting said control voltage to zero in response to said input signal;
determining a braking time according to said current; and
controlling said brake means to apply said braking force at said braking time.